In recent years, gallium nitride-based compound semiconductors represented by the formula AlxGayIn1-x-yN (0≦x≦1, 0≦y≦1, x+y≦1) have become of interest as materials for producing a light-emitting diode (LED) which emits ultraviolet to blue light, or green light. Through employment of such a compound semiconductor, ultraviolet light, blue light, or green light of high emission intensity can be obtained; such high-intensity light has conventionally been difficult to attain. Unlike the case of a GaAs light-emitting device, such a gallium nitride-based compound semiconductor is generally grown on a sapphire substrate (i.e., an insulating substrate); hence, an electrode cannot be provided on the back surface of the substrate. Therefore, both a negative electrode and a positive electrode must be provided on semiconductor layers formed through crystal growth on the substrate.
In the case of the gallium nitride-based compound semiconductor device, the sapphire substrate is permeable with respect to emitted light. Therefore, attention is drawn to a flip-chip-type light-emitting device, which is configured by mounting the semiconductor device on a lead frame and the like such that the electrode face the frame, whereby emitted light is emitted through the sapphire substrate.
FIG. 1 is a schematic representation showing a general structure of a flip-chip-type light-emitting device. Specifically, the light-emitting device includes a substrate 1, a buffer layer 2, an n-type semiconductor layer 3, a light-emitting layer 4, and a p-type semiconductor layer 5, the layers being formed atop the substrate through crystal growth. A portion of the light-emitting layer 4 and a portion of the p-type semiconductor layer 5 are removed through etching, thereby exposing a portion of the n-type semiconductor layer 3 to the outside. A positive electrode 10 is formed on the p-type semiconductor layer 5, and a negative electrode 20 is formed on the exposed portion of the n-type semiconductor layer 3. The light-emitting device is mounted on, for example, a lead frame such that the electrodes face the frame, followed by bonding.
During mounting of a flip-chip-type light-emitting device, a negative electrode is heated to some hundreds of degrees Celsius. Therefore, the negative electrode of a flip-chip-type light-emitting device is required to resist a deterioration, in characteristics, caused by heating.
As an exemplary negative electrode which provides excellent Ohmic contact with a gallium nitride-based compound semiconductor, there has been known such an electrode that is formed through vapor deposition of Al, Cr, and Ti on an n-type gallium nitride-based compound semiconductor layer (see, for example, Japanese Patent Application Laid-Open (kokai) No. 5-291621). However, when heated, the negative electrode has deteriorated characteristics. Another known negative electrode is formed through vapor deposition, on an n-type gallium nitride-based compound semiconductor layer, of an undercoat layer formed of at least one metal selected from the group consisting of V, Nb, Zr, and Cr, or formed of an alloy containing the metal and, on the undercoat layer, a main electrode formed of a metal different from the metal forming the undercoat layer, followed by thermally annealing the formed multi-layer structure (see, for example, Japanese Patent Application Laid-Open (kokai) No. 10-112555). However, as the above method includes a thermal annealing step after formation of a negative electrode, productivity of the electrode is unsatisfactory.